How does attenuation regulate the tryptophan operon in bacteria?

• A. High tryptophan levels cause termination structures to form early, terminating transcription; low tryptophan allows transcription of downstream genes
• B. Low tryptophan promotes early termination; high tryptophan allows complete transcription
• C. Attenuation only responds to lactose levels
• D. Attenuation enhances transcription regardless of tryptophan

Answer: (D)

Attenuation uses the translation of a short leader peptide to decide whether transcription of the operon continues. In the tryptophan operon, the leader region includes tryptophan codons. When tryptophan is abundant, the ribosome moves quickly through the leader, allowing the mRNA to fold into a terminator structure that causes transcription to stop before the structural genes are transcribed. When tryptophan is scarce, the ribosome stalls at those tryptophan codons, which changes the folding so an antiterminator form is favored and transcription proceeds into the operon to produce enzymes for tryptophan synthesis.